The mainstay of the local telephone company network is the local subscriber loop, i.e., the loop from a central office (“CO”) to a subscriber. The local subscriber loop is now being used to provide broadband digital telecommunication services such as digital subscriber line (“DSL”) service. Such broadband DSL services include integrated services digital subscriber network (“ISDN”), high-rate digital subscriber line (“HDSL”), asymmetrical digital subscriber lines (“ADSL”) and very high rate digital subscriber lines (“VDSL”) technology. DSL services allow residential and business customers to send and/or receive digital data at higher rates of speed than were previously possible using analog modem technology.
DSL technologies are engineered to operate over a class of subscriber loops, such as nonloaded loops (18 kft) or Carrier Serving Area (CSA) loops (9 to 12 kft). Digital Subscriber Line (DSL) technology exploits the existing, ubiquitous, copper telephone loop plant to provide megabit per second (Mbps) high-speed Internet access and other services. The great majority of residential customers and many business customers are served by such metallic (copper) twisted pair cables connected from a local switch in the central office (“CO”) to the subscriber's landline telephones. For each subscriber, telephone and DSL signals travel on a twisted pair from a central office (CO) to the subscriber.
There are many impairments to DSL transmission including loop loss and crosstalk. DSL signals are attenuated and distorted by transmission through the loop, particularly at high frequencies and on loops with bridged tap. Some of the power of a DSL transmitting on a loop travels through a crosstalk-coupling path and generates crosstalk noise into other DSLs on loops in the same cable. Additionally, there is impairment from radio ingress and impulse noise, which is sometimes worse than the impairment from crosstalk. Electromagnetic interference (EMI) due to radio ingress appears as narrowband noise spikes in the frequency domain, and impulse noise occurs as brief spikes in the time domain. All these impairments vary in severity by tens of dB's from loop to loop.
Crosstalk generally increases with increasing frequency, and because DSL frequencies extend into the Megahertz (“MHz”) range, crosstalk becomes the major limitation to high-speed ADSL. As time progresses it is expected that there will be many more ADSL users each demanding higher speed service. This will result in more crosstalk and higher-bandwidth services that are more vulnerable to crosstalk. Sources of crosstalk are often called “disturbers.” There are two types of crosstalk: near-end crosstalk (NEXT) and far-end crosstalk (FEXT). NEXT is more powerful than FEXT, particularly below about 1 MHz where many DSLs use overlapping spectra. If there is one crosstalker, then the received crosstalk PSD is the product of a PSD transmitted on a nearby pair times the crosstalk coupling transfer function. With multiple crosstalkers the received crosstalk PSD is the power sum of each component.
If a DSL or other system transmits a power spectral density (PSD) on one pair of a multi-pair cable, then this PSD is multiplied by a crosstalk coupling function in the frequency domain, and the resulting crosstalk couples into a nearby pair. Spectral compatibility is the property that crosstalk between different systems that transmit in the same twisted-pair cable does not significantly degrade the performance of any of the systems. Spectrum management is the process of deploying DSLs in the loop plant in such a manner that ensures spectral compatibility. Current techniques for spectrum management apply rigid rules uniformly across the entire loop plant, as embodied in ANSI T1.417, the Spectrum Management Standard, developed by ANSI-accredited DSL standards committee T1E1.4. These rules do not take into account the individual types of crosstalk sources and crosstalk couplings of a particular cable, which may be considerably different than the near worst-case couplings that are assumed in the spectrum management standard.
DSL lines are typically maintained by using tests developed for POTS lines, which ignore frequencies above 4 kHz. DSL lines that fail because of the environment at high frequencies can sometimes be repaired by knowledgeable technicians with expensive manual tests, or the DSL service may simply be abandoned.
In typical current DSL provisioning the loop working length determines if a customer can get high rate service (˜1.5 Mbps), low rate service (˜400 kbps), or no service. Telephone loops vary considerably at high frequencies, with noise and crosstalk levels typically differing by 20 dB or more on different loops. The achievable bit rates that could be offered to customers are usually significantly higher than those currently provisioned. Moreover, some unexpected service failures are inevitable. DSL modems do self-adapt to their loop, for example by lowering the bit rate if need be. But this does not provide the DSL service provider much specific information or control.
DSL is a relatively new service from the local exchange carriers (LECs). Current practice assumes that there is little knowledge about a particular loop's transmission parameters except a rough estimate of loop length. All DSL services must withstand a statistical worst-case environment, assuming 99% worst-case crosstalk couplings that are only exceeded on 1% of cables, and binders filled with the worst-case types of crosstalkers. This conservative practice denies some customers DSL service that could have otherwise been provided such service (false negatives), in order to achieve a low number of expensive unexpected failures (false positives). However, it fails to completely eliminate false positives, since it does not account for the many different factors that can cause failures such as high levels of radio ingress or impulse noise. Worse, many DSLs are set to transmit higher power than necessary, creating unnecessarily high levels of crosstalk, instead of responding properly to the actual impairments on each particular loop.
Therefore, it would be desirable to have a system for using measurements of crosstalk, electromagnetic noise, background noise and loop make up to precisely determine DSL performance.
Furthermore, it would be desirable to have such a system for the automated identification and isolation of problems that result in degradation of the DSL performance.
Additionally, it would be desirable to implement these methods in a system that could efficiently provision, manage and maintain DSL service even before the installation of any DSL modems.
It would be desirable to provide a system that could automatically diagnose problems with DSL service once such DSL modems were installed.
It would be desirable to provide a system that could enable higher bit rates capable of supporting video transmission using current DSL technology.
Furthermore, it would be desirable to have a DSL management system that can identify potential problems with most DSL lines allowing DSL to be a carrier-grade service with solid service level agreement (SLA) guarantees.
Finally, it would be desirable to have a DSL management that automatically identifies the most costly and difficult to diagnose problems to provide remediation advice (i.e., remove bridged tap) before expending effort in the field.